The invention relates generally to Schottky-barrier semiconductor devices, and uses therefor. More particularly, the invention is directed to Schottky-barrier semiconductor devices for use in measuring strain, temperature and the like.
Methods are known in the art for measuring mechanical strain, which are based on measurements of resistance, the piezoelectric effect, or the acousto-optical effect. Conventional strain gauges vary in speed, sensitivity, bandwidth, system complexity and cost.
Conventional resistive strain gauges have a conductive element, such as a metal ribbon, epoxied to a flexible member. The gauge is included as a component in a balanced Wheatstone bridge resistive network. The strain produced by vibrations transfers through the flexible member to the metal ribbon and alters its resistivity. This fractional change in the resistivity unbalances the bridge producing a current proportional to the strain. The resistive strain gauge is a narrow band device, responding only in a limited bandwidth in the vibrational spectrum. Dynamic calibration of the bridge is a tedious process and integration in sliding contacts--in which it is desirable to measure strain--requires careful packaging.
Some of the disadvantages of the resistive strain gauge are overcome by using a voltage or capacitive method to measure strain. Certain crystals such as quartz and PZT are piezoelectric in nature. Under stress or strain they become polarized, and the polarization appears as a voltage across the crystal, or alternatively as a capacitive charge. The piezoelectric transducer is packaged in a metal fixture that is mated to the vibrating assembly. The voltage produced by the transducer is amplified electronically. However, sensitive piezoelectric crystals are expensive to fabricate, package, and calibrate. Furthermore, each crystal requires its own calibration.
Strain coupled to an acousto-optic material such as quartz, LiNbO.sub.3, or water produces changes in the refractive index of the material. Laser light incident on the material can undergo Bragg diffraction or deflection. The angle of deflection is proportional to the vibrating stimulus, and the deflection is measured on a position-sensitive photodetector array. This is a sensitive method of detecting high frequency vibrations. However, high quality crystals for acoustic sensing are expensive, and the experimental apparatus required for a simple measurement is fairly complex, and unsuitable for measuring strain at multiple locations in a commercial engine, hydraulic system, transmission or the like.
Methods are known in the art for measuring temperature. A common technique used for electrical temperature sensing is to measure the motion of charges in a sensor element. Temperature sensors are commonly constructed as resistors, bi-metal junctions, or semiconductor devices, and require a variety of measurement and calibration techniques.
In resistive temperature sensors, the mean-free path of the electrons between scatter events is related to the temperature. As the temperature increases, the vibratory motion of ions in the resistive material increases, and electrons traveling through the resistive material have a higher scattering probability. The electron mean-free path is reduced and the sensor becomes more resistive. When the temperature declines, the inverse process is true. Resistive sensors often have carbon-glass or metal-film resistors. For example, if aluminum is used as the metal film sensor material, its resistivity changes from 3.55 .mu..OMEGA.-cm at 300.degree. K. to 2.45 .mu..OMEGA.-cm at 273.degree. K. and 0.3 .mu..OMEGA.-cm at 77.degree. K.
Bi-metal junctions are formed when two dissimilar metals make physical contact. A potential barrier is formed at the junctions. Electrons are thermally excited over the barrier, and the current across the barrier is temperature sensitive. Bi-metal thermocouples are most useful at high temperatures.
Semiconductor temperature sensors are usually constructed as p-n junction diodes, bipolar junction or field-effect transistors. The motion of electrons through a crystal is temperature dependent. The crystalline lattice is in a state of vibratory motion in well-defined phonon modes. The phonons scatter electrons randomly. If the temperature of the material is comparable to its Debye temperature, the mode density, governed by Bose-Einstein statistics, is fairly high. The probability of an electron-phonon scatter event is proportionally high, and the electron's mean-free path is relatively short. As the sensor temperature decreases the motion of the crystalline lattice "freezes." Fewer phonon modes are excited in the lattice, and the electron motion is relatively unhindered by any interaction with the lattice. Electrons, may however, undergo Coulomb scattering by a sparse population of impurity ions until the carrier motion itself freezes. This occurs at very low temperatures approaching a few degrees Kelvin.
An alternative technique for temperature sensing in semiconductors is to simply monitor the thermionic emission across the potential barrier in a p-n or a metal-semiconductor junction. The emission probability is proportional to the classical Maxwell-Boltzmann distribution. Sensors using this alternative technique are particularly effective at high temperatures where thermionic emission increases exponentially.
Any of the above-described temperature sensors may be used in a conventional temperature-sensing system as a part of a balanced resistive network powered by a constant-current source. The temperature is measured by monitoring the voltage across a reference resistor held at a fixed temperature. The temperature dependent current through the sensor is deduced from this voltage.
It is known in the art of semiconductor technology that when a semiconductor is brought into contact with a metal, a barrier layer is formed in the semiconductor from which charge carriers are severely depleted. This barrier is known as a Schottky barrier. a Schottky diode is formed in the region where a metal contacts a lightly doped semiconductor. Schottky diodes have a faster response time and lower operating voltage than doped silicon junction diodes. Metal in contact with a highly doped semiconductor (5.times.10.sup.17 atoms per cubic centimeter), however, forms a regular ohmic contact. The Schottky barrier forms because the work function of the metal is greater than the work function of the doped semiconductor, and the metal depletes the semiconductor in the region around the contact of charge carriers, typically electrons, leaving in the semiconductor a depletion layer of positively charged donor ions that is practically stripped of electrons.
Semiconductor devices using the Schottky barrier are known in the art. Soares, S. F., "Photoconductive Gain in a Schottky Barrier Photodiode", Jpn. J. Appl. Phys., Vol. 31, pp. 210-216 (1992), discloses a pair of metal contacts on a lightly-doped n-type semiconductor for use in photodetection. At each interface between metal and semiconductor, a Schottky barrier forms. In "Heterodyne Ultraviolet Photodetection" (Dissertation of S. F. Soares, University of New Mexico, Albuquerque, submitted December, 1989), metal contacts are deposited about 3 microns apart on a lightly-doped silicon substrate, and each has an area in contact with the substrate of about 50 square microns to about 250 square microns. The semiconductor dopant density is selected so that the depletion regions of the two contacts almost extend to each other. It is further known that application of a bias across the pair of Schottky barriers enhances the detection of photo events.
There is a need for a simple, inexpensive, fast, robust strain gauge for inclusion in gears and bearings in aircraft engines and transmissions, for example, and engines and transmissions generally. Resistive gauges are difficult to package and time consuming to calibrate. Piezoelectric crystals are expensive and require complex electronic circuits to detect minute stress-induced voltages. Acousto-optic techniques are based on expensive laser techniques.
There is a further need for a temperature gauge for these same engine environments, especially a gauge that can cooperate with a strain gauge, where strain measurements may be thrown off by changes in temperature due to friction or combustion heat from operation.